Lead acid batteries have four basic components, a positive electrode, which may be a lead or lead alloy grid pasted with a lead oxide positive active material (PAM) coating, a negative electrode, which may be a lead or lead alloy grid pasted with a negative active material (NAM) coating, a separator, and a liquid electrolyte, generally sulfuric acid. To prevent physical contact between the electrodes of opposite polarity while allowing for ionic flow, electrically insulating porous separators are placed between the electrodes. Separators often include a microporous polymer membrane or material, for instance a polyolefin membrane, such as polyethylene (PE) membrane. In a lead acid battery, the area in which the electrolyte and positive electrode meet is called the interfacial “oxidation zone.” The oxidation may be purely chemical or purely electrochemical or a combination of both. This oxidation zone is on the order of several hundred microns or μm and extends into the electrolyte where the separator is placed. Polyethylene and similar polymers are not inherently oxidation resistant during the battery operation, and as such may undergo accelerated oxidation, especially portions of the separator which are located in the oxidation zone. Oxidation of the separator material may lead to reduced battery performance and life span.
A typical separator will often surround either, or both, of the positive or negative electrodes, usually in an envelope, pocket or sleeve configuration. The envelope or sleeve is obtained from a single sheet of separator material which is folded into the shape necessary to surround the electrode. This folding and cutting is often done with automated equipment in a continuous fashion. Over time, automotive battery manufacturers have reduced the backweb thickness of separators from 250 μm to 150 μm, because reducing the volume of the separator allows additional electrolyte and electrode material to be present in the battery, increasing power and performance. However, because bending stiffness is related to the cube of the thickness, even small reductions in thickness can substantially reduce bending stiffness. For instance, a 30% reduction in thickness can result in a 70% decrease in bending stiffness. The reduced separator stiffness presents manufacturing challenges with existing equipment. For instance, reduced stiffness increases the propensity for inadvertent folds and creases, leading to higher rejection rates of the finished separator. Reducing manufacturing speed can reduce rejection rate, however, the productivity loss that accompanies such reduction is often commercially undesirable or infeasible.
When lead acid batteries are deeply discharged the gravity of the electrolyte will decrease as the sulfuric acid is a participant in the energy storage reactions. Upon recharging, pure sulfuric acid, which has higher density than bulk electrolyte, is created at the surface of the electrodes (i.e., a boundary layer). At the boundary layer only the outer portion of sulfuric acid will diffuse into the bulk electrolyte, while the remaining sulfuric acid, because it is heavier than the electrolyte, will collect in the bottom of the battery. This separation of sulfuric acid from bulk electrolyte is termed “acid stratification.” The reduced level of acid at the top of the battery inhibits plate activation and increases corrosion. Also, increased acid concentration at the bottom artificially raises the voltage of the battery, which can interfere with battery management systems. Overall, acid stratification causes higher resistance which leads to shorter battery life.
Hence, there is a need for improved separators and/or batteries. For example, there may be a need for improved separators or batteries that may provide improved or enhanced charge acceptance, surface conductivity, oxidation resistance, wettability, bending stiffness, and/or cycle life, and/or reduced acid stratification, a need for batteries, especially lead acid batteries, with improved charge acceptance and/or reduced acid stratification, a need for battery separators with improved wettability, improved surface conductivity, improved oxidation resistance, and/or increased stiffness, and/or a need for manufacturing processes which allow the rapid production of battery components, including separators, with reduced rejection rates of finished separators.